FIG. 1 provides a perspective view of a conventional commercial airliner 1 having gas turbine jet engines 2. FIG. 2 presents an enlarged view of a cut-away section of the jet engine 2 of FIG. 1, including a fan containment case 4 that surrounds rotary blades 6 of the jet engine 2.
In rare instances, one or more of the blades 6 in the jet engine 2 may be caused to release, for example, as a result of the ingestion of a foreign object. In such an event, the released fan blade must be contained so as not to penetrate the fan case 4. In addition, the fan case 4 must retain its structural integrity while the jet engine 2 shuts down in order to prevent further potentially catastrophic damage. Under these circumstances, a high strength fan containment case becomes critically necessary.
The mechanisms of fan blade release are further illustrated with reference to FIGS. 3-5. FIG. 3 presents a cut-away sectional view of the fan containment case 4 along the lines 3-3 of FIG. 2. Blades 6 are shown at the intake side of the case 4. FIG. 4 presents a front view of the fan containment case 4 looking aft down a longitudinal axis defined by fan hub 8 of the engine 2 at which the blades 6 are secured.
During normal operation of the jet engine 2, the blades 6 rotate around the fan hub 8. Due to the engine rotation, centrifugal forces are generated on each blade 6 that is supported by the fan hub 8. During a blade-out event, blade 6a (as shown in FIG. 5) disengages from the fan hub 8 to become a pointed projectile which can impact the interior of the fan case 4 and cause the generation of a point load at the location of impact. As illustrated in FIG. 5, with the dislodging of blade 6a, a hoop tensile load is generated at the area of impact of the blade 6a. The resulting point load at the interior of the fan case 4 also results in a distortion in the symmetry of fan case 4, as shown by the displacement 13 in the case 4 of FIG. 5.
Two approaches for containing a released fan blade within the fan case 4 have been successfully used previously. In a first approach (the “softwall” fan case), a metal casing is over-wound with dry aramid fibers. A broken blade is allowed to pierce and pass through the metal layer, where it is stopped and contained within the external aramid wrap. In the second approach (the “hardwall” fan case), a single metal hardwall casing is designed to reflect the broken blade back into the engine. The hardwall approach enables designers to improve engine aerodynamics by building a fan case with a smaller radial envelope, since there is no “dead space” required for absorbing the broken blade. However, hard wall fan cases tend to be comparatively heavy, and still maintain some risk that the blade may pass completely through the fan case.
Accordingly, it would be desirable to overcome the drawbacks of prior art methods used for containing fan blades in jet engines during “blade-out” events.